1. Field of the Invention
The present invention relates to improved sealing of subpad edges used for a variety of applications, including semiconductor polishing.
2. Background of the Invention
Silicon wafers are produced as precursors from which microelectronic semiconductor components are produced. The wafers are cut from cylindrical silicon crystals, parallel to their major surfaces, to produce thin disks, typically 20–30 cm in diameter, although larger or smaller wafers are possible. The resulting wafers must be polished to give flat and planar surfaces for deposition of electronic components onto the surface by standard lithographic and etching techniques to form integrated chip semiconductor devices. Typically a 20–cm diameter wafer will produce about 100 or more microprocessor chips.
The designed size of such integrated chips is steadily decreasing, while the number of layers applied, e.g. by lithography onto the silicon surface, is rising to produce ever smaller and increasingly complex microcircuits. Present semiconductors typically incorporate 8 or more metal layers, and it is expected that future designs will contain even more layers. The decrease in the size of circuitry and the increase in the number of layers applied are leading to ever more stringent requirements on the smoothness and planarity of the silicon and semiconductor wafers throughout the chip manufacturing process, since uneven surfaces ultimately result in imperfections and defects leading to unplanned resistances where a conductor is narrowed, to capacitances/nonconductive gaps where breaks occur in deposited conductor layers, and to unplanned short circuits where insulating barriers are breached, which interfere with or compromise the planned operation of the circuit.
In the semiconductor chip fabrication process, it is well-known that there is a need to polish a semiconductor wafer. This polishing is typically accomplished by a chemical mechanical (CMP) process. One standard CMP wafer polishing technique is to remove a wafer from a stack, or cassette of e.g. 25 wafers, by means of a robotic arm, and maneuver the wafer into position over a rotating flat polishing pad mounting on a large turntable. An overhead wafer carrier maintains the wafer in place while being polished by a rotating pad and a chemical-mechanical polishing slurry applied to the surface of the pad. The slurry is generally made up of an aqueous solution with metallic or non-metallic particles such as, for example, aluminum or silica abrasives that create added friction for the polishing process. The polishing pad is usually made of polyurethane. This is an adaptation of optical polishing technology used for polishing lenses, mirrors, and other optical components. Once polishing is completed, the robot arm removes the wafer and transfers it to another workstation for eventual lithographic deposition and etch steps.
A significantly different approach is the so-called Linear Planarization Technology (LPT), wherein a traveling belt is used to polish the wafer, in place of the rotating flat turntable form of polishing tool. The belt used in this method is described in EP-A-0696495 and comprises a belt of sheet steel or other high strength material, having a conventional flat polyurethane polishing pad affixed to it with adhesive. As with the rotating pad, the pad used for LPT CMP polishing receives a chemical-mechanical polishing slurry that is distributed over the surface of the belt.
The polishing pads, discussed above, are often stacked onto compressible subpads in order to increase the ability of the pad to conform to the wafer surface during polishing. Unlike the polishing layers of the pads or belts, the subpads are typically liquid absorbent. And when liquid (e.g., the CMP slurry) soaks into the subpad, the subpad's physical properties change, which in turn changes or impairs the polishing performance of the polishing layer stacked onto the subpad.
One common way that liquid (e.g., the CMP slurry) can contact, and thereby soak into, the subpad is through the edges of the subpad. These edges include the edges around the outer periphery of the subpad as well as the edges of an internal aperture in the subpad, such as is used for endpoint detection.
Attempts to inhibit liquid absorption into the subpad edges were made in WO 01/15864 (Freeman et al.), where the edges of the subpad were heat sealed, pressure embossed, or coated with a waterproof substance such as silicone rubber. In U.S. Pat. No. 6,123,609 (Satou), the subpad surface and edges were completely covered with a polishing web, which was attached directly to the rotating turntable with waterproof tape.
Those strategies, while helpful, are not without their disadvantages and drawbacks. One significant drawback is that the seals and/or adhesives used in these strategies are exposed to attack by the CMP slurry, which can degrade the seals and/or delaminate the polishing layers. Another disadvantage is that the belt or pad user must perform the sealing operation at the site of use, rather than having the belt manufacturer preseal the subpad edges. Finally, the edge sealing of the prior art is unsuitable for continuous polishing belts which subject the edge seals to pronounced stretching and bending forces as the belt is pulled around the spinning rollers. The excessive bending and stretching compounded with the high stresses and rotational speed of the belts often lead to cracks and fractures in the edge sealant through which the CMP slurry may enter the subpad.